ADVANCED EXPERIMENTAL PHYSIOLOGY 375 



It has been shown that a bright object on a dark background appears, 

 when suddenly exposed, to be surrounded with a red border lasting a 

 fraction of a second. If the illumination be brighter a blue green effect 

 is visible. These facts in part explain the appearance of colours shown 

 when the discs below are rotated. 



EXPERIMENT VI. Rotate the discs (Figs. 245 and 246) and note the 

 coloured fringes or areas appearing according to the direction. The 

 rate must be slow. The simple fusion of sensation corresponding to 

 intermittent stimuli rapidly repeated may be also shown in the Experi- 

 ments I. and II., Section E of the Milton Bradley Pseudoptics. 



FIG. 246. 



4. The Fusion of Colour Sensations. Several methods have been 

 devised with the object of enabling us to fuse separate colour sensations. 

 These depend either upon separate colours forming images on the 

 retina in such rapid succession as to be inseparable, or else upon 

 separate colours forming images in the same portion of the retina so 

 that the sensations are super imposed. 



The first method is generally carried out by means of the separate 

 colours being arranged as sectors in a circle, which is rapidly revolved 

 about its centre, the instrument adapted for the purpose being known 

 as a colour-mixer. Discs of different colours, such as the Wundt series, 

 are obtainable, and each disc has a radial slit at one point so that these 

 can be arranged upon a common centre and a circle may be made up of 

 sectors of various discs. It is desirable to have discs of two sizes, one 

 about ten inches across, the other four or five inches. It is to be 

 remembered that these discs are not coloured with pure colours of the 



